1. Field of the Invention
The invention relates to the field of MOS integrated circuits, and particularly to the formation of resistor elements in such circuits.
2. Prior Art
In the early years of metal-oxide-semiconductor (MOS) technology, resistance elements for integrated circuits were provided by diffused regions in the substrate, polysilicon members, and the like. These elements, which occupied relatively large areas in the circuit, are not as widely used with the advent of more complex circuitry requiring higher densities.
The lack of adequate resistors for use in high-density semiconductor integrated circuits led to an avoidance of their use. Circuits were deliberately designed to use fewer resistors, and transistors were, in many cases, used as load devices in place of resistors. The static memory cell, for instance, has traditionally been constructed as a six-transistor bistable circuit, in which two of the six transistors serve as load devices.
Resistor elements utilizing ion-implanted regions have been described in U.S. Pat. No. 4,246,692 (implanted regions buried beneath field oxide), U.S. Pat. No. 4,110,776 (implanted resistors over field oxide), U.S. Pat. No. 4,209,716 (implanted resistors in second-level polysilicon) and U.S. Pat. No. 4,330,931 (polysilicon and tungsten composition members). The closest prior art examples known to Applicant are the vertically-oriented buried polysilicon resistor element described by Yoshio Sakai et al, 1984 Symposium on VSLI Technology Digest of Technical Papers, p. 6-7, September 1984, and the ion implanted polysilicon resistor element described in U.S. Pat. No. 4,416,049 and the plasma enhanced chemical vapor deposition described by A. C. Adams, VLSI Technology, pp. 93-129, edited by S. M. Sze, McGraw-Hill, 1983.
Each of these prior art techniques presents associated difficulties. The conductivity of polysilicon creates a need for relatively large polysilicon resistor elements, since a relatively long pathway is required to achieve the desired resistance. Many of the prior art techniques require critical masking steps to achieve the necessary precision in the length and width of the polysilicon load. Use of polysilicon loads can also give a high surface contour, leading to fracturing of the films on the final circuit. The high diffusivity for boron or phosphorus dopants in polysilicon lends additional difficulties to the use of high-resistive polysilicon regions as load devices.
The present invention represents a departure from previous technologies. This invention uses a plasma enhanced chemical vapor deposition silicon-rich nitride film as a contact-window-load device. This film is not polysilicon, although it could have micro-polysilicon inter-mixed with nitride, and presents a number of advantages over polysilicon for use as a resistor element.